[PATCH][next] wifi: brcmfmac: fweh: Fix boot crash on Raspberry Pi 4

[Gustavo A. R. Silva]

Fix boot crash on Raspberry Pi by moving the update to `event->datalen`
before data is copied into flexible-array member `data` via `memcpy()`.

Flexible-array member `data` was annotated with `__counted_by(datalen)`
in commit 62d19b358088 ("wifi: brcmfmac: fweh: Add __counted_by for
struct brcmf_fweh_queue_item and use struct_size()"). The intention of
this is to gain visibility into the size of `data` at run-time through
its _counter_ (in this case `datalen`), and with this have its accesses
bounds-checked at run-time via CONFIG_FORTIFY_SOURCE and
CONFIG_UBSAN_BOUNDS.

To effectively accomplish the above, we shall update the counter
(`datalen`), before the first access to the flexible array (`data`),
which was also done in the mentioned commit.

However, commit edec42821911 ("wifi: brcmfmac: allow per-vendor event
handling") inadvertently caused a buffer overflow, detected by
FORTIFY_SOURCE. It moved the `event->datalen = datalen;` update to after
the first `data` access, at which point `event->datalen` was not yet
updated from zero (after calling `kzalloc()`), leading to the overflow
issue.

This fix repositions the `event->datalen = datalen;` update before
accessing `data`, restoring the intended buffer overflow protection. :)

Fixes: edec42821911 ("wifi: brcmfmac: allow per-vendor event handling")
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Closes: https://gist.github.com/nathanchance/e22f681f3bfc467f15cdf6605021aaa6
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
 drivers/net/wireless/broadcom/brcm80211/brcmfmac/fweh.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/fweh.c b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/fweh.c
index 0774f6c59226..f0b6a7607f16 100644
--- a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/fweh.c
+++ b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/fweh.c
@@ -497,12 +497,12 @@ void brcmf_fweh_process_event(struct brcmf_pub *drvr,
 		return;
 
 	event->code = fwevt_idx;
+	event->datalen = datalen;
 	event->ifidx = event_packet->msg.ifidx;
 
 	/* use memcpy to get aligned event message */
 	memcpy(&event->emsg, &event_packet->msg, sizeof(event->emsg));
 	memcpy(event->data, data, datalen);
-	event->datalen = datalen;
 	memcpy(event->ifaddr, event_packet->eth.h_dest, ETH_ALEN);
 
 	brcmf_fweh_queue_event(fweh, event);
-- 
2.34.1